This invention relates to electrically traced conduits (that is, pipes, tubes or the like which carry fluid), and in particular to a novel means for carrying such a conduit through a wall which separates a highly hazardous area from a less hazardous area.
Electrically traced conduit is widely used, for example, in process industries, to maintain the temperature of a fluid in the conduit at or above some critical temperature. The critical temperature is generally the freezing point of the fluid, but may also be, for example, the condensation temperature of a gas or the temperature at which the viscosity of a liquid becomes undesirably great.
Electrically traced conduit may take many forms. The electrical heat tracer has traditionally been a mineral insulated heating cable, the heat flux of which is dependent on the length of the cable. More recently, heat tracers have been produced in the form of a strip which includes a pair of conductors separated by a resistor material. Because the parallel conductors carry full line voltage throughout their length, the heat output per foot is independent of the length of the cable. If the resistor material has a positive temperature coefficient, the heater strip becomes self-limiting and will not exceed a predetermined temperature even if it is wrapped across itself. Such self-limiting parallel-circuit heating tape is commercially available by itself or as part of a "bundle" which includes one or more conduits and one or more heating tapes surrounded by insulation and enclosed in a water-impervious jacket. When a preinsulated bundle is not used, insulation must be added around the conduit after it is traced.
Because parallel circuit heating tape carries full line voltage throughout its length, it must be provided with a metallic armor, properly grounded, in any environment in which flammable or explosive vapors may be present. In environments where flammable or explosive vapors may normally be encountered, the use of parallel-circuit heating cable is presently prohibited. Such areas are classified as Class 1, Division 1 areas by the National Electric Code and are designated "hazardous areas" herein. When a conduit is traced with an electrical heat tracer which is not approved for hazardous areas and when that conduit extends into a hazardous area, certain additional difficulties arise which have not heretofore been resolved in any fully satisfactory manner.
Although the presence of a hazardous fluid in a conduit does not in itself render the area around the conduit hazardous, electrically traced conduits may well extend into an area which is designated hazardous. One situation in which a traced conduit may extend into a hazardous area arises when the flow, volume or pressure of a hazardous fluid must be measured in an environment where the temperature may drop below the freezing point of the fluid. For example, the flow of a hazardous fluid through a pipe may be measured by determining the pressure differential across an orifice plate in the pipe. Impulse lines communicating with the pipe on opposite sides of the orifice plate lead to a meter which derives a flow indication from the relative pressures in the impulse lines. In such installations, both the impulse lines and the meter may need to be freeze-proofed. The impulse lines are protected by electrical tracing and insulation. The meter can be protected by enclosing it in a heated insulative enclosure, such as a foamed polyurethane box. Because the meter includes a number of manually operable valves with their associated packing and other connections which are routinely loosened and tightened, it is to be expected that during its life a certain amount of leakage will occur. The interior of the meter enclosure may therefore be considered to be a hazardous area, and all electrical wiring, including heating elements, in the enclosure may be required to be housed in protective explosion-proof containers. Prior to the present invention, no effective means has been known for providing a transition between the traced lines outside the enclosure and the interior of the enclosure. If the tracer and insulation are spaced from the enclosure, the lines can freeze. If they are brought up to the wall of the enclosure, they become effectively a part of the enclosure and present a danger. Furthermore, no provision has previously been made for supporting and protecting the lines, and more importantly the tracer and any electrical connections to the tracer. It will be appreciated that generally the same problem has been encountered whenever an electrically traced conduit, traced with a non-approved heat tracer, extends through a wall separating a hazardous area from a less hazardous area, and the general problem has been equally lacking a satisfactory solution.